Hangups in Safari and Firefox

Increasingly we are experiencing the "beachball of doom" while surfing with either browser. The only way to escape is to force-quit the application. Any idea what's going on here? This is an iMac running Snow Leopard and otherwise seems to work fine.

The iMac has 1 GB of memory. Not sure about broadband speed, but it's Comcast cable. Also have a 250 GB hard drive and a 500GB external hard drive backing up on Time Machine. The issue seems to show up when the external drive is active?? Like maybe when it's backing up?

If you are downloading and browsing, not a good time to have the external on and backing up, particularly with so little memory. Personally always drag external icon to the trash and disconnect when not in use.

Hang on to those original install discs like grim death! Using OS X.7 or later make a bootable USB thumb drive before running Installer!